Pickling apparatus



April 1969 'D. R. EDWARDS 3,441,035

PICKLING APPARATUS Filed Nov. 7, 1966 United States Patent 3,441,035PICKLING APPARATUS Derrik Robert Edwards, St. Annes-on-the-Sea, England,assignor to United Kingdom Atomic Energy Authority, London, EnglandFiled Nov. 7, 1966, Ser. No. 592,557 Claims priority, application GreatBritain, Nov. 19, 1965, 49,249/ 65 Int. Cl. B08b 3/04; F16k 19/00 US.Cl. 134-103 4 Claims ABSTRACT OF THE DISCLOSURE Apparatus for thepickling of metallic objects comprising an elongate pickling vessel andmeans for causing oscillatory turbulent fiow of treatment liquid to andfro along the pickling vessel and over the metallic objects containedtherein while maintaining the pickling vessel full of treatment liquid.

This invention relates to apparatus for performing pickling processes inwhich metallic objects are treated with an etchant liquid usually anacid, so that the outer surface layer is removed by chemical action, toleave a clean cocntamination free surface.

The efficiency of any pickling process can be related to severalfactors, the most important of which are the etching of material fromthe surface being pickled at a uniform rate over the whole surface,involving good heat transfer and gas dispersion at the surface beingtreated, together with efiicient mixing of the etchant liquid. Inaddition the speed with which the etching process can be stopped, oncethe desired amount of material has been removed from the object beingpickled, is also important. Together these factors under idealconditions give very precise control over the whole pickling operationsuch that the desired amount of material is removed and at the same timepitting and contamination of the cleaned surface is avoided.

In conventional types of pickling apparatus a series of horizontallydisposed baths are normally employed. The metallic objects to be pickledare totally immersed in one of these baths containing etchant liquid,the latter being agitated, by propellers or pump circulators forexample, to ensure adequate mixing while rotation or oscillation of theobjects being pickled may also be provided. When sufficient material hasbeen removed from the objects undergoing pickling, the etching action isstopped by the rapid transfer, using mechanical means, of the objects toanother bath containing liquid capable of rapidly stopping the etchingaction. Further washing of the pickled objects can be provided bytransfer to other baths. Other types of pickling apparatus include theDi-phase pickling tank in which etchant and stop solutions occupy thesame tank as two separate phases, separated by an interphase. Provisionis made for mixing within each phase and for the passage of the objectsbeing pickled directly from the etchant phase to the stop phase. Furtherwashing of the pickled objects can be provided by transfer to othertanks. Collectively however known pickling plants possess disadvantageswhich allow only inadequate control over the pickling operation becauseof the slow transfer between the etchant and stop sections, andinadequate mixing of the etchant liquid. In addition they often requireelaborate safety schemes to protect operating personnel from toxic andsplash hazards and also employ complex mechanical linkages which areliable to corrosion by the etchant liquid.

It is an object of this invention to provide apparatus suitable for theefficient pickling of metallic objects which will obviate thedisadvantages of known pickling plants.

According to the present invention apparatus for the pickling ofmetallic objects comprises an elongate pickling vessel, a number ofstorage tanks for containing treatment liquids, means for filling thepickling vessel with liquid from any one of said storage tanks and meansfor causing oscillatory turbulent flow of liquid to and fro along thelength of the pickling vessel, while maintaining the pickling vesselfull of liquid.

In a preferred arrangement the pickling vessel is in the form of avertically arranged tube, means being provided for connecting any one ofthe storage tanks with the lower end of the pickling vessel, a surgetank being connected with the upper end of the pickling vessel and meansbeing provided to force liquid from a storage tank to fill the picklingvessel and to cause turbulent oscillatory flow of liquid upwards anddownwards in the pickling vessel by cycling of liquid from the storagetank through the pickling vessel into the surge tank and then back fromthe ksurge tank through the pickling vessel into the storage tan Fuelfor water cooled nuclear reactors may comprise clusters of nuclear fuelelements; each element consisting of a stack of uranium dioxide fuelpellets enclosed in a sheath of zirconium or zirconium alloy. It hasbeen established that under operative conditions in a water-coolednuclear reactor, the work hardened and contaminated surface layer formedon fuel element cans of zirconium or zirconium alloy during fuel elementmanufacture, is considerably less resistant to corrosion by hightemperature water and steam than the base metal. It is thereforenecessary for this layer to be removed before the fuel element clustersare loaded into the nuclear reactor.

A pickling apparatus embodying the invention and capable of processingclusters of nuclear fuel elements canned in zirconium or zirconium alloywill now be described with reference to the accompanying drawing showinga schematic arrangement of the apparatus in side elevation.

The apparatus consists of a vertical 8 inch bore cylindrical picklingvessel 1 constructed of polypropylene reinforced with fibre-glass, apartfrom an upper load bearing section 2 of stainless steel lined withpolypropylene and equipped with a polypropylene support ring 2a and aremovable lid 3 also lined with polypropylene for loading and unloadingthe pickling vessel 1 with a nuclear fuel element cluster 4. Each fuelelement cluster typically consists of'36 fuel elements each 13 feet inlength, attached at their upper ends to a circular perforated stainlesssteel plate, equipped with a lifting eye, and spaced apart at theirlower ends by a perforated polypropylene grid. Support for the nuclearfuel element cluster being provided by the location of the stainlesssteel perforated plate on the prolypropylene support ring 2a in theupper load bearing section 2 of the pickling vessel 1, the support ring2a being provided with channels permitting liquid flow to and from thepickling vessel 1. The lower end of the pickling vessel 1 is connectedby linked piston operated valves 5 and 6 and polypropylene piping tothree mild steel polypropylene lined storage tanks 7, 8 and 9 containingrespectively etchant acid, demineralised water for washing the fuelelement cluster 4 after etching to stop the etchant action, and adetergent composition in demineralised water used to degrease the fuelelement cluster 4 before the pickling process commences. The upper loadbearing section 2 of the pickling vessel 1 connects to a mild steelpolypropylene lined surge tank 10 equipped with high 11 and low 12liquid level detectors. The linking of the valves 5 and 6 ensures thatonly one storage tank 7, 8 or 9 can be on line with the pickling vessel1 and the surge tank 10 at any one time. Solenoid operated gas valves13a and 13b respectively control the supply and exhaust of compressedair or other suitable gas such as nitrogen to and from the storage tanks7, 8 and 9 and the surge tank 10. A gas/liquid interface detector 14 ispositioned on the line leading from the valve 5 to the etchant liquidstorage tank 7. The plant is arranged so that all the storage tanks areat a lower level than the pickling vessel ensuring that the latter isself draining, and that treatment liquids are not in contact with valves5 and 6 unless the plant is operational. Only one valve 5 is exposed toattack by the etcliant acid and that only for the duration of theetching cyc e.

In operation the admission of compressed air or other suitable gas suchas nitrogen through the appropriate gas supply value 13a to the upperpart of any one of the storage tanks 7, 8 or 9 above the liquid/ gasinterface in the tank causes liquid to be forced out of the tank to fillthe pickling vessel 1. The liquid overflows from the pickling vessel 1into the surge tank 10 has its gas valve 1312 open to exhaust. When theliquid overflowing into the surge tank 10 reaches the level of the highliquid level detector 11 it causes the storage tank gas supply valve 13ato shut and its gas exhaust valve 13b to open while the surge tank 10gas exhaust valve 131) closes and the gas supply valve 13a opens toadmit compressed air or other suitable gas such as nitrogen into theupper part of the surge tank 10 above the liquid/gas interface in thesurge tank 10. This causes the liquid in the surge tank 10 to be forcedback down the pickling vessel 1. When the liquid in the surge tank 10reaches the level of the low liquid level detector 12 it actuates thereversal of the gas valves 13a and 13b of both the storage tank and thesurge tank such that the valve 13a in the storage tank again admitscompressed air or other suitable gas such as nitrogen into the storagetank and the valve 13b in the surge tank 10 is again connected toexhaust. Thus liquid is again forced upwards through the picklingvessel 1. Repetition of this cycle causes an oscillatory turbulent flowof liquid upwards and downwards in the pickling vessel 1 over a fuelelement cluster 4 which is contained therein. The duration ofoscillation of the liquid in the pickling vessel 1 can be pre-set byautomatic equipment or can be manually controlled.

A typical sequence of treatment cycles employed in the pickling ofzirconium or zirconium alloy canned fuel element clusters will now begiven:

Duration, mins.

(1) Detergent wash (alkaline decreasing in demineralised water at 50 C.)5-10 (2) Demineralised water wash-cold at least5 (3) Demineralised Waterwash-cold at leastS (4) Etchant liquid wash (2% of azeotropehydrofluoric acid and 20% concentrated nitric acid in demineralisedwater at 30 C.) -2l0 (5) Stop-demineralised water washcold 2-5 (6)Demineralised watercold at least5 (7) Demineralised watercold at least 5sel and then rapidly actuates valves to cycle the stopdemineralisedwater wash for a pre-set time. I

The pickling apparatus described minimises handling damage to theobjects being pickled and lends itself to the employment of a variety ofcombinations of etching, stop and wash cycles, of variable duration andin addition during the etching cycle the oscillatory turbulent flowprovides good mixing, heat transfer and gas dispersion resulting in analmost uniform surface removal rate. These factors coupled with a veryrapid transfer from etching to stop cycles, and the absence of any toxicand splash hazards for the operating personnel combine to provide anefiicient pickling apparatus.

I claim:

1. Apparatus for the pickling of metallic objects comprising an elongatepickling vessel for receiving said objects, a number of storage tanksfor containing treatment liquids, means for filling the pickling vesselwith liquid from any one of said storage tanks and means for causingoscillatory turbulent w of liquid to and fro along the length of thepickling vessel, while maintaining said pickling vessel full of liquid.

2. Apparatus for the pickling of metallic objects as claimed in claim 1wherein the pickling vessel is in the form of a vertically orientatedtube, provided with means for connecting any one of the storage tankswhile the lower end of the pickling vessel, and a surge tank connectedto the upper end of the pickling vessel, means being provided to forceliquid from any one storage tank to fill the pickling vessel and tocause oscillatory turbulent flow of liquid upwards and downwards in thepickling vessel.

3. Apparatus for the pickling of metallic objects as claimed in claim 2wherein means are provided for the admission of gas under pressure tosaid storage tanks and to said surge tank above the level of liquidtherein, oscillatory turbulent flow of liquid from any one of thestorage tanks to and fro along the length of the pickling vessel beingestablished by the admission of gas under pressure into the storage tankabove the level of liquid therein, to drive liquid from the storage tankalong the length of the pickling vessel into the surge tank, the liquidthen being driven from the surge tank back along the length of thepickling vessel by the admission of gas under pressure into the surgetank above the level of liquid therein.

4. Apparatus for the pickling of metallic objects as claimed in claim 2,wherein the storage tanks are set at a lower level than the picklingvessel wherein the pickling vessel is self-draining into the storagetanks.

References Cited UNITED STATES PATENTS 720,337 2/1903 Emery 134-991,195,542 8/1916 Raymond. 2,008,839 7/1935 Samson 13422 2,145,540 1/1939'Ellis. 2,519,654 8/1950 Heaney 134103 2,771,892 11/1956 St. Palley13457 3,118,456 1/1964 St. Palley 13457 ROBERT L. BLEUTGE, PrimaryExaminer.

U.S.Cl.X.R.

